revlog: make clearcaches() more effective

clearcaches() was added several years ago in e8d37b78acfb as part
of implementing a perf command. Since revlog instances have many caches
and since the spirit of this mostly unused method is to facilitate
performance testing, I think it's appropriate for all the revlog's
caches to get cleared when it is called.

mercurial/revlog.py

@@ -339,6 +339,11 b' class revlog(object):'
             return False
 
     def clearcaches(self):
+        self._cache = None
+        self._basecache = None
+        self._chunkcache = (0, '')
+        self._pcache = {}
+
         try:
             self._nodecache.clearcaches()
         except AttributeError: